1. Field of the Invention
The present invention relates to an image sensing apparatus and a control method for the image sensing apparatus, and more particularly, to an image sensing apparatus in which an electronic shutter function and a mechanical shutter function are used in combination to carry out photography, and a control method for the image sensing apparatus.
2. Description of the Related Art
There are some singe-lens reflex digital cameras in which a focal-plane shutter that is a mechanical shutter and an electronic shutter are used in combination to carry out photography. In this type of shutter mechanism, the mechanical shutter constitutes a rear curtain, and photography is carried out by, prior to traveling of the rear curtain, driving the electronic shutter for scanning pixels of an image sensor to start charge accumulation.
In a case in which the electronic shutter is used to carry out photography, for example, in an image sensor using a CMOS sensor, reset scan is first carried out for setting the amount of accumulated electric charge in pixels for each pixel or for each region composed of a plurality of pixels (for example, for each line) to zero. After that, scan for reading out signals after a predetermined period of time for each pixel or each region subjected to the reset scan is carried out, thereby photography using the electronic shutter can be achieved.
In a case in which the above-described electronic shutter and mechanical shutter are used in combination to control exposure of the image sensor to light, reset scan is first carried out sequentially for each region composed of a plurality of pixels of the image sensor (for example, for each line) in the traveling direction of the mechanical shutter as scan of the image sensor for starting charge accumulation. Then, after a predetermined period of time, the image sensor is sequentially shielded from light using the rear curtain of the mechanical shutter, and reading scan is then carried out for sequentially reading out electric charge accumulated in each pixel. Therefore, the scan pattern for this reset scan is adapted to traveling characteristics of the rear curtain of the mechanical shutter (see, for example, Japanese Patent Laid-Open No. 11-41523)
Further, Japanese Patent Laid-Open No. 2006-101492 proposes an image sensing device in which a scan pattern for starting exposure is corrected using a difference between a first image, for which both start and end of exposure are controlled using an electronic shutter, and a second image, for which scan for starting exposure is carried out using the electronic shutter while the exposure is ended using a mechanical shutter.
Singe-lens reflex digital cameras are generally capable of connecting to exchangeable photographing lenses, and the focal length and the exit pupil distance (the distance from an image plane to an exit pupil position of the lens) vary depending on the attached photographing lenses. Therefore, in a case in which an image sensing apparatus is configured so that an electronic shutter and a mechanical shutter are used in combination to carry out photography as described above, the following problems arise. Specifically, the electronic shutter functions on the plane of the image sensor, whereas the mechanical shutter is located a little away from the plane of the image sensor in the direction of an optical axis. Accordingly, the position on the plane of the image sensor for the mechanical shutter providing a shield from light varies depending on the focal length, the exit pupil distance, and the like of the photographing lens. Therefore, particularly when the exposure time from reset scan to shielding from light by the mechanical shutter is short, unevenness of exposure will arise in the traveling direction of the shutter depending on the focal length, the exit pupil distance, and the like of the attached photographing lens. Since it is not possible for a photographer to notice this unevenness of exposure before photography, there is a problem in that the photographer will notice the unevenness when checking a photographed image.
Therefore, in an image sensing apparatus in which an electronic shutter and a mechanical shutter are used in combination, further improvement is needed in order to reduce unevenness of exposure in the scan direction of the shutter, which varies depending on the type of a lens.